Physical security monitoring systems are typically signaled by magnetically actuated switches that include magnetic field sensors mounted on a fixed member of a barrier and actuating magnets mounted on a moveable member of the barrier. The magnetic field sensors may be mounted, for example, on the fixed frame of a doorway, while the corresponding actuating magnets are mounted on the moveable door. When the barrier is secure, the actuating magnets induce a predetermined response at the sensor output from which the magnetic switch signals the monitoring system that the barrier is secure. When the barrier is breached, the actuating magnets are withdrawn from the proximity of the sensor, and the switch signals the monitoring system that the barrier has been penetrated.
Such magnetically actuated switches should be resistant to defeat by foreign magnetic fields. Prior systems have used multiple magnetic field sources of particular magnitude or polarity to elicit a "secure" status from an electric circuit. Such systems have made defeat more difficult, but not impracticable.
For example, U.S. Pat. No. 4,349,814 to Akehurst, describes a machine safety switch that is controlled with two Hall effect magnetic sensors arranged coplanarly and oriented so that each sensor responds to magnetic fields of opposite polarity. Such systems can be defeated however, by appropriate placement of a bar magnet of appropriate dimension close to the sensor plane.
Other prior systems arrange multiple magnets with multiple sensors to create unique magnetic "codings." The apparatus disclosed in U.S. Pat. No. 4,210,889 to Holce describes an arrangement of multiple magnetically responsive reed sensors biased to sense whether corresponding magnetic field sources of predetermined characteristics are either too close or too far from the sensors. Thus, a particular proximal arrangement between moveable and fixed members of a barrier elicits a "secure" signal from the Holce system while an alarm is generated when the moveable and fixed barrier members are situated either too close together or far apart. A later system, U.S. Pat. No. 4,806,910 to Salzer describes an arrangement of several magnets of different field strengths positioned on, or adjacent, a moveable member of a closure mechanism. The multiple field strengths, in combination, purportedly create a system in which a variety of defeat strategies can be detected. The use of multiple electronic sensors, as in Salzar, is limited however, by the extra current demanded by each sensor added to the system.
In addition to having immunity to alien magnetic fields, magnetically actuated switches should be false alarm resistant. Environmental effects, such as ubiquitous building vibration, generally cause irregular and transitory variations in the magnet-to-sensor distance that can initiate false alarms in magnetically switched security systems. The biasing of the reed sensors shown in U.S. Pat. No. 4,210,889 to Holce assists in mitigating such false alarm sensitivity. The biasing of the reed switches makes the switch less sensitive to small variations in magnet to sensor distance. However, some systems using multiple sensors, such as that shown in U.S. Pat. No. 4,806,910 to Salzer, may improve resistance to foreign magnet defeat but may concurrently display decreased immunity to false alarms because of heightened sensitivity to magnetic field changes.
Other prior systems have discriminated between false alarms and unauthorized entry by requiring that the magnet-to-sensor distance vary for more than a predetermined period of time. Such a prior art system is exemplified by U.S. Pat. No. 4,812,674 to Sue in which a fault condition must have a duration longer than the time constant of an RC circuit before an alarm will be triggered. Sue also uses multiple sensors to increase defeat resistance, but as in U.S. Pat. No. 4,806,910 to Salzer, increased sensors require increased current.
All of the previous systems also suffer from the disadvantage of requiring that an operator manually calibrate the cooperative action of the sensors and magnets, thus increasing the security risk for the eventual installation site.